Mechanisms of Cloud-Radiation Interaction in the Tropics and Midlatitudes

Abstract

Radiative forcing and latent heat associated with precipitation are the two most important diabatic processes that drive the circulation of the atmosphere. Clouds can affect radiation and vice versa. It is known that longwave radiative processes can enhance precipitation in cloud systems. This paper concentrates on determining the relative importance of three specific longwave radiative mechanisms by comparing cloud-resolving models with and without one or more of these processes. Three of the ways that longwave radiation is thought to interact with clouds are as follows: 1) cloud-top cooling and cloud-base warming may alter the thermal stratification of cloud layers, 2) differential cooling between clear and cloudy regions might enhance convergence into the cloud system, and 3) large-scale cooling could change the environment. A two-dimensional version of the Goddard Cumulus Ensemble model has been used to perform a series of sensitivity tests to identify which is the dominant cloud-radiative forcing mechanism with respect to the organization, structure, and precipitation processes for both a tropical (EMEX) and a midlatitude (PRE-STORM) mesoscale convective system. The model results indicate that the dominant process for enhancing the surface precipitation in both the PRE-STORM and EMEX squall cases is the large-scale radiative cooling. However, the overall effect is really to increase the relative humidity and not tie convective available potential energy (CAPE). Because of the high moisture in the Tropics, the increase in relative humidity by radiative cooling can have more of an impact on precipitation in the tropical case than in the midlatitude case. The large-scale cooling led to a 36% increase in rainfall for the tropical cast. The midlatitude model squall with a higher CAPE and lower humidity environment was only slightly affected (8%) by any of the longwave mechanisms. Our results also indicated that the squall systems' overall (convective and stratiform) precipitation is increased by turning off the cloud-top cooling and cloud-base warming. Therefore, the cloud-top cooling-cloud-base warming mechanism was not the responsible cloud-radiative mechanism for enhancing the surface precipitation. However, the circulation as well as the microphysical processes were indeed (slightly) enhanced in the stratiform region by the cloud-top cooling and cloud-base warming mechanism for the midlatitude squall case. For both cases, the model results show that the mechanism associated with differential cooling between the clear and cloudy regions may or may not enhance precipitation processes. However, this mechanism is definitely less important than the large-scale longwave radiative cooling. Solar heating was run from 0900 to 1300 LST in both environments and was found to decrease the precipitation by 7% in each case compared to the runs with longwave radiation only. This result suggests that solar heating may play a significant role in the daytime minimum/nighttime maximum precipitation cycle found over most oceans.